Conventionally, condenser microphones are frequently used in mobile phones, for example. A typical construction of condenser microphones is shown in FIG. 7. This condenser microphone comprises a metal capsule 100 including a plurality of perforations “h” corresponding to acoustic holes formed therein, a fixed electrode 300 and a diaphragm 500 provided inside the capsule to be opposed to each other with a spacer 400 therebetween to maintain a predetermined gap, a substrate 600 fixed and fitted to a rear opening of the capsule 100, and an impedance converting element 700 made of J-FET or the like and mounted to the substrate 600. With this type of condenser microphone, a high voltage is applied to a dielectric material formed on the fixed electrode 300 or the diaphragm 500 to be heated to generate electric polarization and produce an electret membrane allowing a residual electric charge to remain on a surface thereof (an electret membrane 510 is formed in a diaphragm body 520 made of metal or conductive film which constitutes the diaphragm 500 in FIG. 7), thereby to provide a construction that requires no bias voltage. When the diaphragm 500 is vibrated by sound pressure signals of a sound, a distance between the diaphragm 500 and the fixed electrode 300 is changed to vary capacitance. The variation of capacitance is outputted through the impedance converting element 700.
A technique for miniaturizing the condenser microphone is known from Patent Document 1 listed below, for example. With this technique, an oxide layer (2), polycrystal silicon layers (3) and (5), a silicon nitride layer (4) and a sacrificial layer made of polycrystal silicon are formed on a silicon wafer (1), and a diaphragm (silicon nitride layer (4)) is formed on the silicon wafer by etching or the like. A back plate having numerous perforations (30) corresponding to acoustic holes and acting as a back electrode is formed on the same silicon wafer (1) by the same technique for forming the diaphragm. The diaphragm and the back plate are superimposed and combined to each other using the technique of eutectic soldering, capacitive coupling, silicon fusion or the like to constitute a unit acting as the microphone (the reference numerals are derived from the reference document).
Further, a technique for miniaturizing the condenser microphone is also known from Patent Document 2 listed below, for example. This technique comprises a first step for forming a mask for forming a recess and doping boron for forming a diaphragm on the back side of a monocrystal silicon substrate (101), a second step for forming a mask for doping boron for forming a back plate on the front surface of the monocrystal silicon substrate, a third step for doping a predetermined amount of boron from the front surface and the back surface of the monocrystal silicon substrate, and a fourth step for forming acoustic holes by dry etching, forming a gap between the back plate and the diaphragm by alkali etching and finally forming an electrode, thereby to complete the microphone. With this technique, the diaphragm (102) and the back plate (103) corresponding to a back electrode are integrally formed with the substrate (101) (the reference numerals are derived from the reference document).
A similar technique is also known from Patent Document 3 listed below, for example. With this technique, a bulk silicon layer (1), an insulating layer (2) and a body silicon layer (3) are laminated. A doping area (8) formed on the body silicon layer (3) is used as a back electrode, and a plurality of openings (10) corresponding to acoustic holes are formed on the doping area (8). A membrane (7) consisting of a membrane layer (5) formed in a position opposed to the doping area (8) through a spacer (4) (sacrificial layer) is used as a diaphragm. With this technique, as with the technique described in Patent Document 2, hollows (9) are formed in the body silicon layer (3) to form the openings (10), and a void (6) is formed between the doping area (8) and the membrane (7) by processes such as mask forming, doping, etching and the like (the reference numerals are derived from the reference document).
Patent Document 1: Patent Publication No. 7-50899
Patent Document 2: Patent Publication No. 2002-95093
Patent Document 3: U.S. Pat. No. 6,140,689